My 2002 Wrx will not boost above 15psi. I had no problems with this until recently. In 1st gear when I hit 15psi the boost won't go any higher. On all other gears when I hit 15psi the boost again won't go any higher but the engine shakes a bit. It feels just like boost cut but I do not get any CELs. If I ease off the gas or shift to a higher gear everything is normal again. Other than that, the car drives and idles perfectly.

This started happening about a month ago. Since then I have had the head gasket replaced because it was leaking. I had a small exhaust leak fixed on the y-pipe. I cleaned the Maf sensor. I reset the ecu. I have emptied and filled the gas tank 3 times with 91 octane fuel. I have added octane booster and fuel system cleaner to the tank. I checked the fuel filter and it's clean. There doesn't appear to be any vacuum leaks.

The ecu map sets the target boost level at 16.5 +/- .7psi. Could something be bypassing the ecu's levels? I checked the boost controller and it's two-port and it is electric so I assume it's going back to the ecu for control. The wastegate actuater can't be adjusted since it's an IHI turbo. The restrictor hose came with the VF-30 turbo but should I get a restrictor pill with a larger diameter hole? Should I just remove the pill? This problem is very specific since it only happens when I reach 15psi. Anyone have any ideas?